Other Parts Discussed in Thread: UCC256402,
Tool/software:
I am using this controller in my LLC converter design, but have issues starting up.
Problem: No gate pulses are produced, when the HO tries to go high, LO does not and there is a shoot through condition.
Test setup:
1) Have tried both methods of powering on the IC i.e, supplying DC bus to HV pin and directly connecting an external power supply to VCC - the IC does turn on because RVCC = 12V, HB - HS = 12V (boot capacitor is charged) and the LO is high (but not only a pulse, it is constantly high)
2) I am not using OVP so I grounded BW. BLK pin is at 3 V from the DC bus through a resistor divider.
- I have also tried to externally supply ISNS at 3 V since the internal OCP1 threshold is 4.03 V, just to avoid overcurrent tripping.
- VCR is also being supplied from a function generator (4Vpp sine wave at 90 kHz) just for debugging and trying to generate HO and LO pulses. Capacitance at this pin to ground is 15nF.
3) FB pin voltage is sometimes 5V, but after some time (not consistent) when the HO tries to go high the LO does not. Other times, FB is low, and I cannot generate any switching pulses.
I do have a LL/SS capacitor to ground of 0.22uF. Is this an issue? Maximum suggested capacitance on this is 120pF in some design sheets from TI.
